I think the mystery has been exacerbated by the way RJ explained things. I've been trying to take his statements and boil them down--i think it may help to understand what he meant if we try to re-word his explanation while preserving his meaning.

I've come to boil it down as follows: By saying it is intuitively obvious, RJ is saying that the answer pops out -- it comes to you -- it is a passive revelation, not a mystery solved.

He shows us how the identity pops out.

He placed one person at the scene of the murder.

If you identify the person who was at the scene you have identified the murder.

He said that we should determine every person's location at the time of the murder.

He told us how to determine everyone's location. We need to determine where the timing of events puts everyone at the time.

So, take the timing of events and map it out. Who is situated at the scene OR consider people moving from point A to point B crossing paths at the time and place.